Evaluating the response of winter cultivars of rapeseed (Brassica napus L.) to different levels of salinity stress

Introduction
The effect of salinity stress on plants is very wide, and it can cause a decrease in growth, damage to the roots, decrease in yield and even death of plants by reducing the absorption of water and nutrients, reducing enzyme activity, and disrupting the physiological processes of plants such as photosynthesis and respiration (Gupta and Hong, 2014 and Awaad, 2023). Rapeseed oil, as one of the most important sources of vegetable oils in the world, is very important in various economic, agricultural, industrial, and health sectors. With the ability of growing in different climatic conditions, having low water requirement and good relative resistance to various environmental stresses, rapeseed is known as a suitable plant for crop rotation (Raboanatahiry et al., 2021). The effects of salinity on the characteristics of the rapeseed plant in the stages of germination, vegetative stages and seed ripening have been proven. Considering that different rapeseed cultivars are different from each other in terms of their susceptibility to salinity stress in different stages, a detailed assessment on the effects of salinity stress on rapeseed and the adoption of appropriate strategies to manage and reduce this stress can help in better cultivation and optimal management of rapeseed; Therefore, evaluating the response of different rapeseed cultivars to salinity stress is of particular importance in order to introduce salinity-tolerant cultivars
 
Methodology
Experimental model
In order to evaluate the response of winter rapeseed (Brassica napus L.) cultivars to different levels of salinity, an experiment was conducted in a greenhouse at the Research Greenhouse of Payame Noor University of Gandaman (Chaharmahal and Bakhtiari province). The experiment was arranged as a split-plot design within a completely randomized design with three replications. The first factor was four levels of salinity, including S0 (2), S1 (10), S2 (20), and S3 (30) (dS/m), where S0 was the Hoagland's solution as the control, and other salinity levels were the result of mixing sodium chloride (NaCl) and calcium chloride (CaCl2) at a ratio of 20 to 1 moles in the Hoagland's solution, respectively. The second factor comprised eight winter rapeseed cultivars including (Licord, Okapi, SLM 046, Modena, Opera, Symbol, Fornax and Elite). The experimental units included 40 × 40 cm pots, 35 cm in height, containing completely homogenized washed sand. Due to lack of absorption and less the plants' need for a complete nutrient solution, the pots were irrigated every other day with a solution containing half the concentration of nutrients found in the Hoagland's solution (Dehdari et al, 2005). Two weeks after establishment, the plants were thinned from 15 to 8 plants per pot to achieve the desired plant density. From the 20th day after sowing (the four-leaf stage of the plants), salinity levels were gradually applied to acclimate the plants, such that all pots, except for the control level, received incremental salinity levels of 25% of each level in four irrigation shifts, thereby applying the total salinity treatment for each level.
 
Collecting and preparing plant samples
The salinity treatment continued at the specified ratios until the end of the growth stage. In this study, the traits of seed yield, dry matter traits, thousand seed weight, oil yield and oil percentage were measured. At the end of the growth stage (90 days after sowing), to measure the seed yield, dry matter traits, thousand seed weight and oil yield, six plants in each pot were randomly selected, harvested, and the above parameters were measured and calculated. Oil percentage was calculated with a Soxhlet apparatus (Joshi et al., 2008).
 
Statistical Analysis Statistical Analysis
 Finally, data were analyzed using SAS software (version 9.1). Mean comparisons were conducted using Duncan's multiple range test at a 5% probability level.
 
 Results and discussion
In the present study, all the traits under investigation were affected by salinity. An increase in salinity levels across all evaluated cultivars resulted in a significant reduction in all examined traits. Based on the results, it can be inferred that the rapeseed cultivars evaluated in this study can produce an acceptable yield up to a salinity level of S1 (salinity of 10 dS/m). At this salinity level, the cultivar SLM 046, with a seed yield of 22.78 grams per pot and an oil yield of 7.97 grams per pot, was identified as the most successful cultivar. Conversely, the Okapi cultivar, with an average seed yield of 11.99 grams per pot and an oil yield of 3.71 grams per pot, was the most susceptible cultivar. Therefore, according to the findings of this study, the SLM 046 cultivar was determined to be the most tolerant to salinity at all levels tested, while the Elit cultivar was assessed as the most susceptible to salinity. Since the primary goal of producing canola oilseeds is oil production, cultivating the evaluated cultivars under salinity conditions of S3 (salinity of 30 dS/m) and higher is practically ineffective.
 
Conclusions
Exposure to environmental stressors, including salinity and drought stress, directly and indirectly causes a portion of the assimilates produced by the plant to be diverted from growth and production processes towards coping with and mitigating stress conditions (Ahmed & Umar, 2011). It has been stated that with increased salinity stress, the stressed plant experiences reduced activity and degradation of cellular biomolecules, lipid oxidation, protein structure alteration, enzyme deactivation, chlorophyll bleaching, and nucleic acid degradation, resulting in the limitation of the plant's photosynthetic system (Ahmed & Umar, 2011; Al-Sharari et al., 2023). In the present study, increasing the intensity of salinity stress from S0 (control) to S3 (30 dS/m) caused the highest percentage reduction in all evaluated traits, including seed yield and oil yield, across all tested cultivars. The results indicated that the SLM046 cultivar, with the highest harvest index (0.29) and seed yield (10.52 grams per pot) at the highest salinity level (S3), and the lowest percentage reduction in harvest index (25.64%) compared to the control, performed better in mitigating the effects of salinity stress compared to other cultivars. In salinity stress conditions, salinity-tolerant cultivars like SLM046, which can allocate remaining assimilates effectively despite limitations in phloem transport and new physiological demands due to salinity stress, are more successful. By maintaining an appropriate harvest index and directing resources towards economic yield, these cultivars produce satisfactory yields (Ahmed & Umar, 2011; Khalid et al., 2015).